Audio, video and other signal information can be conveyed over a radiofrequency transmission channel as coded digital signals. Satellite digital telebroadcasting such as that defined in the Digital Video Broadcasting-Satellite (DVB-S) European specification based on MPEG compression standards, for example, is used to convey signal information in the form of digital video signals using quadrature digital modulation. The television signals originate from the satellite and are amplified and converted into a predetermined frequency band (typically 950-2150 MHz) by a parabolic dish and a low-noise converter that is located at the focus of the parabolic dish.
The signal is conveyed to the input of a tuner of a receiver. The purpose of the tuner is to select the desired channel and to output a baseband signal on an in-phase path (I path) and on a quadrature path (Q path). The signal is then converted into a digital signal and demodulated. Decoding includes distinguishing the zeros from the ones and performing error correction, typically utilizing Viterbi decoding, deinterlacing, Reed-Solomon decoding, and deshuffling. A decoding device typically outputs packets that are decoded in a conventional manner in accordance with the MPEG standards so as to redeliver as output the initial audio and video signals transmitted via the satellite.
At the input of the receiver, the signal received is composed of the collection of channels which are transmitted by the satellite and are transposed into the 950-2150 MHz frequency band. The overall power received is substantially equal to the mean power on a channel increased by ten times the Napierian logarithm of the number of channels. This signal possesses a considerable variation, one on the order of 50 dBm.
At present, in receivers as a whole, the signal received at the input is normally filtered by a wide band type filter (whose passband is of the order of several hundred MHz) placed just after the low-noise input amplifier, this being to avoid the saturation of the subsequent stages of the tuner (especially the controlled-gain amplification stages as well as the mixers of the frequency transposition stage). Furthermore, with conventional devices the low-noise input amplifier and the wide band filters are not part of the chip containing the controlled-gain amplification stage as well as the frequency transposition stage. The filters, whose cutoff frequencies can be tailored by selecting the desired channel, are embodied as discrete components such as “varicap” diodes.
Such components are relatively oversized, which is incompatible with a fully integrated embodiment of the tuner. Not only is the tuner not fully integrated, but, moreover, it is made on a semiconductor substrate different from the substrate which supports the digital part for processing, namely the demodulation and the actual channel decoding. In other words, the demodulation and the channel decoding are carried out in a separate component from that integrating the tuner. Also, the tuner is generally shielded so as to prevent the noise generated by the digital part from interfering in the mixing of the signals of the analog part.
Thus, at present, a front-end device incorporated into a satellite television signal receiver, and capable of performing the tuning, demodulation and channel decoding, comprises several separate electronic components made on different chips. This arrangement poses several disadvantages among which are added cost and use of considerable surface area.